This invention relates to the recovery of oil from subterranean formations by water flood operations and more particularly relates to the use of divalent metal ions and chemical additives in surfactant flooding operations.
Petroleum fluids are usually recovered from a subterranean formation or reservoir by employing the natural energy of the reservoir to push the petroleum fluids from the reservoir into wellbores. The technique is referred to as primary recovery. However, 65% to 90% or more of the original oil is usually left in the reservoir at the conclusion of the primary recovery program. When the natural reservoir energy is unable to produce petroleum fluids at an economically determined rate, it is conventional to employ enhanced oil recovery techniques in order to recover more of the petroleum fluids remaining in the reservoir.
One such enhanced oil recovery technique employed is a water flood operation. A water flood operation uses injection wells drilled into the reservoir to inject flood water into the reservoir. The flood water increases the pressure in the reservoir and initially provides additional energy to push oil into production wellbores for recovery. The flood water replaces the oil that is initially produced and forms a wave front traveling through the reservoir towards the production wellbores. The water wave front displaces at least some of the oil in the reservoir and drives or carries this oil through the formation into production wellbores for recovery. Water flood operations can achieve advantageous recovery of oil when subsurface geological conditions are favorable.
However, even when geological conditions favor a water flood operation, oil recovery may be limited due to the relatively poor ability of the flood water to displace oil remaining in the reservoir's pore structure. This is due to several factors including the pore size of the reservoir, its structure, and oil viscosity. The pore size and structure of the reservoir along with the oil resisting dispersion in the water may make the pore structure impermeable to the oil, while the pore structure is permeable to the flood water. So the flood water, having less flow resistance than the oil, may form channels or fingers, bypassing the oil in the formation. For this reason, a large portion of the oil usually remains in the reservoir after a water flood operation.
Investigations of ways to increase oil recovery by improving the displacement ability of a water flood have produced useful surfactants which reduce the interfacial tension between the oil and water in the reservoir. With lower interfacial tension, oil that was trapped in the pore structure can disperse into the water as smaller and more easily deformable droplets. Many types of surfactants have been investigated and the choice of which surfactant to employ in a water flood operation is dependent upon the conditions in the reservoir, as well as the cost and availability of the surfactants.
U.S. Pat. No. 3,366,174 to Ferrell et al. discloses a water flood surfactant composition of a low to medium weight petroleum sulfonate and an oil-soluble organic polar material. U.S. Pat. No. 3,508,612 to Reisberg et al. discloses a water flood surfactant composition of an organic sulfonate and a sulfated oxyalkylated alcohol. Various other mixtures of anionic and nonionic surfactants are sometimes employed in water flood operations. U.S. Pat. Nos. 3,811,504; 3,811,505; 3,890,239, are representative of such mixtures of anionic and nonionic surfactants used in water flood operations.
Most water flood operations have employed a petroleum sulfonate as a sole surfactant, or at least a major component of a mixture of surfactants. Synthetic alkylaryl sulfonates and alkyl sulfonates and sulfates have been proposed as oil recovery surfactants.
Field water flood operations employing an organic sulfonate surfactant have not always been totally satisfactory due to the fact that surfactants sometimes separate from the water and form a non-miscible layer on top of the water. This layer does not move in conjunction with the water wave front, resulting in loss of oil recovery activity and a quantity of surfactant. Further, when organic sulfonates contact divalent metal ions, which are almost always present in underground hydrocarbon formations, the organic sulfonates tend to precipitate out of the water flood and plug the reservoir pores. As a result, the prior art suggests avoiding divalent metal ions in the surfactant system, and especially in the drive fluid.
To combat layering and precipitation problems, a material with both water-soluble and oil-soluble characteristics is usually added to organic sulfonate surfactant mixtures. When used in water flooding, these materials are generally referred to as "solubilizers" and often constitute the most expensive component in a surfactant mixture. Conventional solubilizers are sulfate or sulfonate salts of polyethoxylated alcohols or alkyl phenols. The amount of solubilizer required depends on the amount of and types of organic sulfonate surfactants employed in the water flood operation. A minimum amount of solubilizer is required to prevent the surfactants from layering out of the flood water. Surfactant quantity is in turn a function of the reservoir's size and other characteristics.